1. Field of the Invention
The present invention relates to an automobile with a brake device and, in particular relates to an automobile of a type in which a plurality of wheel tires are driven in common.
2. Conventional Art
A brake device is an indispensable mechanism for an automobile. An automobile driven by an internal combustion engine generally has being used a so called a friction type brake device in which a hydraulic pressure generated through a brake pedal pressing force by a driver is transmitted to a brake mechanism (which is generally a friction brake mechanism) mounted near respective wheel tires via hydraulic pressure pipings to obtain a braking force there through friction between braking members while converting the kinetic energy into thermal energy.
On the other hand, in an automobile such as an electric car and a hybrid car which uses an electric motor as a drive power source or uses an electric motor and an internal combustion engine jointly as a drive power source, a dynamic braking device or a regenerative braking device is used in parallel with the friction type braking device in which the electric motor serving as the drive power source is caused to operate as an electric generator and converts the kinetic energy to electric energy, and the generated electric power is caused to be absorbed in a resistor or a battery so as to obtain a braking force, however, even in this instance the friction type braking device plays a major roll in the entire braking device.
In a conventional friction type braking device for an automobile, a brake drum or a brake disk serving as a part of the brake members and being attached to a wheel body of the wheel tire is generally exposed as it is so as to enhance heat radiation thereof.
As a result, since the brake mechanism is mounted near the wheel tire, for example, during a bad weather, rain, snow and mud splashed by the wheel tires hits to the brake members and intrudes onto the friction faces of the brake members which changes the friction coefficient of the friction faces, thereby, generation of a braking force corresponding to the brake hydraulic pressure is prevented, and the braking force is unstabilized.
Therefore, under such bad condition it is possible that the braking force actually provided reduces below a braking force to be generated based on the inherent braking hydraulic pressure, moreover, it is difficult or almost impossible to predict the generation of the braking force shortage including amount of the shortage.
As a result, for example, in a case of automatic braking mode during traffic jam in which a braking is automatically actuated while following the movement of a vehicle ahead and in a case of emergency braking which requires a precise control, the braking distance can exceed an allowable range which increases possibility of rear-end collision and puts the vehicle under an extremely dangerous condition.
Further, the conventional braking device requires the hydraulic pipings to the respective braking mechanisms for the respective wheel tires, the structure thereof complexes and the weight therefor increases which raises the manufacturing cost thereof.
In order to overcome the above problems, an automobile in which a friction braking mechanism is provided at a common revolution shaft (a propeller shaft) for transmitting torque from a drive power source such as an internal combustion engine to respective wheel tires has been proposed as disclosed, for example, in JP-A-6-40315 (1994) and JP-A-10-114260 (1998).
The above conventional art does not necessary take into account a stable braking force generation which poses a problem with regard to reliability.
Namely, in the conventional art, because of the provision of the braking device at the common revolution shaft, braking forces (or braking torque) for the respective wheel tires are transmitted via a differential gear mechanism, therefore, if any abnormality is generated in this differential gear mechanism, the braking function is possibly damaged and the problem with regard to reliability is caused.
Further, when any abnormality is caused in the differential gear mechanism, the driving force transmission from the drive power source is also affected and the running function of the vehicle is possibly damaged.
Further, when the braking device is provided at the common revolution shaft, both the frequency of torque transmission and amount of torque to be transmitted through the differential gear mechanism increase which particularly causes a problem.
Still further, when the braking force is not transmitted, fail safe function is lost which again causes a serious problem.
The present invention is carried out in view of the above problems, and an object of the present invention is to provide an automobile which is designed to have a fully reliable differential gear mechanism and to generate always a stable braking force without being affected by environment concerned.
The above object is achieved by an automobile of a type in which a differential gear mechanism including at least a ring gear and a drive gear for revolution drive in a planetary gear is used and a drive power source and a plurality of wheel tires are torque-coupled via a common revolution shaft, wherein the ring gear and the drive gear for revolution drive in the planetary gear are constituted by two series of gear mechanisms of first and second gear mechanisms having a same reduction gear ratio but having different diameters each other and an engagement play clearance between the ring gear and the drive gear in one of the two series of gear mechanisms is differentiated from an engagement play clearance between the ring gear and the drive gear in the other of the two series of gear mechanisms.
Further, the above object can be achieved when a braking device is further provided at the above common revolution shaft and is always used as a brake for stopping the running of the automobile.
Still further, the above object can be also achieved when the above braking device is formed into a common unit with a speed change gear (a transmission) disposed between the revolution shaft and a drive power device.
Still further, the above object can be also achieved through a provision of a torque distribution unit which distributes braking torque generated from the braking device.
Moreover, the above object can be also achieved when the braking device is constituted by a dynamic braking device which also serves as an electric motor for driving the automobile.
With the above structure, even when an abnormality is caused in one of the two gear mechanisms of the differential gear mechanism, a fail safe function is activated in which the torque transmission is simply shifted to the other gear mechanism, and the torque transmission function including the braking torque is not damaged, thereby, a higher reliability is always obtained.
As a result, during running under a bad weather or on a bad conditioned road, the braking device is prevented from being affected from such as splashed water and mud from the wheel tires, accordingly, a braking force instability which is caused when the friction coefficient of the friction faces of the braking device changes is prevented, and a stable braking capacity can always be obtained under any environmental conditions concerned, thereby, an automobile excellent with regard to reliability and safety can be obtained.
Further, since the braking device is collectively located, the pipings for operating medium such as hydraulic pressure oil is simplified and light-weighted which reduces the manufacturing cost thereof.
Preferably, when the above braking device is provided at the speed change gear, for example, when a braking function is provided for a multiple disk clutch in the speed change gear, the braking device can be collected into a single unit which simplifies the structure thereof.
Further preferably, with the provision of the torque distribution unit which distributes braking torque generated from the braking device to the respective wheel tires, in a case when a unique braking force is required such as when road surface friction coefficients at right and left side wheel tires are different and when individual unique braking forces have to be provided for respective four wheel tires when the automobile is turning along a curved road, a braking torque is caused to be generated during the braking operation from the braking device as well as the torque distribution unit is activated to distribute the braking torque properly to the respective wheel tires, thereby, a possible occurrence of dangerous locking at the front wheel tires alone or at the rear wheel tires alone is prevented, and a braking force can always be obtained effectively from the running road face. Further, such as the clutch mechanism and the differential gear mechanism can be utilized as the torque distribution unit.
Still further preferably, through the provision of the electric motor at the speed change gear which regenerates energy during deceleration, for example, in a case of braking from a high speed travelling, namely, in a case when it is judged that energy conversion efficiency to be regenerated by the electric motor being operated as a generator is large, the dynamic braking is used in parallel with the friction braking to generate a braking torque due to conversion resistance when converting a kinetic energy of the vehicle into an electric energy, and the generated braking torque is distributed by the torque distribution unit so as to provide properly distributed braking forces to the respective wheel tires.
In this instance, when the converted electric energy is stored such as in a battery, the stored electrical energy can be reused, for example, as a driving force by supplying the electrical energy to the electrical motor during driving, thereby, an effective energy use can be achieved.
Now, when it is judged that the energy conversion efficiency through the generator operation of the electric motor is small, the braking force is provided from the braking device.